The belt-driven continuously variable transmission is adapted to change a speed ratio continuously by varying a groove width of a pulley holding a driving belt therebetween to displace an effective diameter position of the driving belt. A primary pulley and a secondary pulley of the transmission are individually comprised of a fixed sheave and a movable sheave. A speed ratio in the belt-driven continuously variable transmission thus structured is changed continuously by displacing an effective diameter position of the driving belt by hydraulically reciprocating the movable sheave to vary a groove width of the pulley. Specifically, a main role of the primary pulley is to control the speed ratio. For this purpose, the primary pulley is provided with a primary hydraulic chamber, and the effective diameter position of the driving belt in the primary pulley is controlled by controlling hydraulic fluid delivered to the primary hydraulic chamber to actuate the movable sheave. Meanwhile, a main role of the secondary pulley is to control a torque transmitting capacity. For this purpose, the secondary pulley is provided with a secondary hydraulic chamber, and a clamping pressure for clamping the driving belt is controlled by controlling hydraulic fluid delivered to the secondary hydraulic chamber. An example of this kind of belt-driven continuously variable transmission is disclosed in Japanese Patent Laid-Open No. 2009-2414.
The belt-driven continuously variable transmission taught by Japanese Patent Laid-Open No. 2009-2414 is comprised of a hydraulic chamber to which hydraulic fluid is delivered to push a movable sheave of a pulley, and a driving torque cam mechanism and a reversely driven torque cam mechanism disposed between a movable sheave of a secondary pulley and an output gear. The driving torque cam mechanism is comprised of an input side cam member fixed to the movable sheave of the secondary pulley, and an output side cam member integrated with the output gear. The driving torque cam mechanism thus structured is adapted to create a thrust force for pushing the movable sheave of the secondary pulley toward the fixed sheave by a relative rotation between the input side cam member and the output side cam member when the belt-driven continuously variable transmission is transmitting the driving force in the direction to propel the vehicle in the forward direction. Meanwhile, the reversely driven torque cam mechanism is comprised of an output side driven cam member fixed to the movable sheave of the secondary pulley, and an input side driven cam member integrated with the output gear. The reversely drive torque cam mechanism thus structured is adapted to create a thrust force for pushing the movable sheave of the secondary pulley toward the fixed sheave by a relative rotation between the input side driven cam member and the output side driven cam member when the belt-driven continuously variable transmission is transmitting the driving force in the reverse direction.
Meanwhile, international publication WO2010/021218 discloses a hydraulic control system comprised of a supply-side control valve for delivering hydraulic fluid to primary and secondary chambers, and a drain-side control valve for draining the fluid from those chambers. Therefore, the hydraulic fluid is confined in those primary and secondary chambers by closing those control valves.
According to the belt-driven continuously variable transmission taught by Japanese Patent Laid-Open No. 2009-2414, the driving belt is clamped by a total of the thrust force created by the driving torque cam mechanism or the reversely driven torque cam mechanism, and the thrust force of the movable sheave created by the hydraulic fluid delivered to the secondary hydraulic chamber. As described, however, the movable sheave is reciprocated to displace the effective diameter position of the driving belt. That is, in order to create a thrust force by the driving torque cam mechanism, a contact between the input side cam member and the output side cam member have to be always maintained irrespective of reciprocating motion of the movable sheave. Likewise, in order to create a thrust force by the reversely driven torque cam mechanism, a contact between the input side driven cam member and the output side driven cam member have to be always maintained irrespective of reciprocating motion of the movable sheave. Therefore, the torque cam mechanism of this kind has to be large enough to push the movable sheave certainly, and this may enlarge a size of the belt-driven continuously variable transmission.